1. Field of the Invention
The present invention relates to a storage control apparatus and a method thereof for actually accessing a physical storage device constituting a logical unit according to the access request from the host in the units of the logical unit, and more particularly to a storage control apparatus which constitutes a large logical unit by concatenating a plurality of logical units, and a method thereof.
2. Description of the Related Art
In a storage device using such a storage medium as a magnetic disk, magneto-optical disk and optical disk, the storage medium is actually accessed by the request of the data processor. If the data processor uses large capacity data, a storage system comprising a plurality of storage devices and a control apparatus for controlling the storage devices is used.
In such a storage system, the host recognizes the physical disk unit in the units of the logical unit (or logical volume). Depending on the capability of the control apparatus, the capacity of the logical unit is limited. FIG. 12 shows a block diagram of a prior art storage system, and FIG. 13 is a diagram depicting the concatenation of conventional logical units.
As FIG. 12 shows, the host 100 is connected to the control apparatus 110. The control apparatus 110 is comprised of a plurality (4 in this case) of CAs (Channel Adapters) 120, 122, 124 and 126, and a plurality (2 in this case) of CMs (Centralized Modules) 128 and 130.
The CAs 120, 122, 124 and 126 are connection modules with the host 100, and the CM 128, 130 includes a cache memory and controls the disk. The CM 128, 130 controls the logical unit 140, 150. The logical unit 140, 150 is comprised of a single or a plurality of physical disk devices which constitutes the logical volume.
For the connection format of the CM 128, 130 and the logical unit 140, 150, the maximum number of logical units (number of physical disks) that can be connected to one CM is limited, considering the load distribution specified depending on the capability of the controller of the CM 128, 130 and the size of the cache memory, and one logical unit is not connected to a plurality of CMs.
In other words, in order to improve the response of host access, a cache memory is disposed in the CM 128, 130, and stores a part of the data of the disk device which the respective CM charges, and the write data from the host.
In other words, when read access is received from the host, the data of the cache memory is read and transferred without actually accessing the disk apparatus if the target data is staged in the cache memory. When write access is received from the host, the write data is written to the cache memory, and the writing is completed. The write data in the cache memory is written back to the target disk device during processing idle time.
If the cache memory is installed to improve host access in this way, the load must be distributed with limiting the number of logical units to be connected to one CM, since the size of the cache memory is limited, and the capability of the controller is also limited.
However, some users have the demand to increase the capacity of the logical unit when viewed from the host side. Therefore conventionally a method for concatenating a plurality of logical units 140 and 142 to be connected to one CM 128, so that the host 100 recognizes them as one large logical unit LU0, has been provided, as shown in FIG. 13.
With this prior art, however, a plurality of logical units cannot be concatenating under one CM, so the load of the CM for controlling the concatenated large capacity logical unit increases, which may drop host access performance.
Also in the case of a system which has a plurality of CMs, the logical units to be concatenated must be under one CM, so the number of logical units to be concatenated is limited, and constructing a flexible large capacity logical unit is difficult.